The benefits from building a hydro dam on the Mokihinui River, north of Westport, are obvious.

It would, by using a resource that on the West Coast is endlessly renewable, give the region enough electricity to power 45,000 homes.

The dam would not only supply most of the region's electricity needs in an undeniably carbon-zero way, it would also end the reliance on a long and vulnerable transmission line that brings the area's present power supply from the Waitaki. Supply would not only be more secure, it would be more efficient and West Coast electricity prices, at present some of the highest in the country, would be lower.

According to Meridian Energy, the promoter of the scheme, it has the "overwhelming support" of locals. One of them is the Mayor of Buller, Pat McManus, who is an enthusiastic supporter of the idea. He says that the "silent majority" of local people support it. He acknowledges that it would have an impact on the environment but says that the area that would be affected is only a "very small footprint" of the total West Coast area. The local iwi, Ngati Waewae, is also pleased with the scheme. Through its chairman, Francois Tumahai, the iwi has said it is satisfied that the project adequately addresses cultural effects through mitigation measures including maintaining passage for native fish and undertaking pest control. He has welcomed the prospect of the Coast becoming self-sustainable in electricity.

Despite this, the decision this week of hearing commissioners by a vote of two to one to approve the proposal will cause controversy. Although the scheme has attracted little attention among the wider public until now, green groups are stridently against the dam. Inevitably, the objections arise from the environmental impact of the scheme. The proposal would require a 85-metre high, 300m wide dam across the river that would create a narrow, 14 kilometre long lake covering 340 hectares. Meridian says that the impact would be minor and it has made a considerable effort to make sure they are kept to a minimum. No endangered species are threatened, it says. In addition, the resource consents Meridian has received have more than 200 conditions attached to them to further reduce the impact. Nonetheless, according to the objectors, a precious, irreplaceable part of the landscape will be irretrievably changed.

The idea that untouched areas should be left that way is one that has substantial appeal to many middle-class voters. The fact that the Revenue Minister and United Future leader, Peter Dunne, has joined the objectors is an indication of this. Dunne has never been particularly visible on green matters before but he has a reasonably astute awareness of where public opinion lies.

But the country cannot afford to have decisions like this one made on emotion and sentiment. Electricity demand is growing by 2 per cent a year, equivalent to the needs of a city the size of Dunedin. The two-to-one vote on this scheme shows that the commissioners' approval was not easily arrived at but it was made, as it must be under the Resource Management Act, after rigorously detached consideration of all the arguments. In this case the commissioners decided the development's impact on the environment are not bad enough to block the project.